(1) Field of the Invention
The present invention relates to an emission cleaning installation for cleaning and eliminating emissions, and to an emission cleaning method which uses this emission cleaning installation, and more particular to an emission cleaning installation which can be fitted into existing large-scale incineration furnaces and is able to oxidatively break down environmental pollutants which are contained in the emissions expelled from an incineration furnace and of which dioxins, dibenzofurans, coplanar PCBs, nitrogen oxides (NOx) and the like are representative examples, by the photocatalytic action of a photocatalyst, without control of the emission temperature, and still more particular to an emission cleaning method which uses this emission cleaning installation.
(2) Background of the Related Art
Hitherto, emission cleaning installations which involve cleaning the emissions expelled from incineration furnaces have been proposed as follows:    1. A Cleaning installation whereby the emission supply duct is provided with electrostatic separators or bag filters or cyclones and the like, in order to physically eliminate dioxins from the emissions;    2. A Cleaning installation which is filled with an adsorption agent comprising activated carbons, activated charcoals and the like, in order to eliminate by adsorption environmental pollutants of which the dioxins contained in the emissions are representative;    3. A Device which mixes photocatalysts for breaking down environmental pollutants in the emissions by oxidation, by means of the photocatalytic effect of the photocatalyst with the emissions, and introduces them into an emission decomposition tower, and which recovers the photocatalyst by means of dust separators downstream of the emission decomposition tower after the photocatalyst has been photoexcited by means of ultraviolet light emitted from a low-pressure mercury lamp, thus revealing its photocatalytic effect (patent application JP-H05-285342).
However, the emission cleaning installation which is described in patent application JP-H05-285342 is an installation which mixes the photocatalyst into the emissions, and if a large amount of photocatalysts is mixed into the emissions, the transmission through the emissions deteriorates (because the light is reflected, and consequently the light transmission deteriorates). Although it is possible for exciting light which is radiated from the light source to be radiated to a sufficient extent with respect to the photocatalyst situated relatively close to the light source, and therefore the photocatalyst can be photoexcited, thus revealing the photocatalytic action (=photocatalytic function), the exciting light (which is emitted from the light source) cannot be radiated to a sufficient extent with respect to the photocatalyst situated at a distance from the light source. Therefore, because the photocatalyst cannot be sufficiently photoexcited, the photocatalytic function of these photocatalysts cannot be sufficiently displayed and, overall, it is known that there are problems with the efficiency of the emission cleaning method brought about by the photocatalytic action (=photocatalytic function) of the photocatalyst being reduced considerably.
There were also further problems relating to the cleaning installation overall becoming complicated and therefore more expensive, since the photocatalyst had to be separated back out of the decomposition gas after treatment, for example by means of separators and the like.
On the other hand, there are elimination problems with devices provided with electrostatic separators or bag filters or cyclones and the like and there are also problems with eliminating and recovering dioxins and the like in the emissions using such devices. Specifically, of the dioxins contained in the emissions, although it is possible to eliminate the dioxins contained in fly ash, together with dust materials, the dioxins adhering to very fine fly ash low-boiling, gaseous dioxins and the like cannot be eliminated.
Furthermore, since the emissions expelled from incineration furnaces are generally at elevated temperatures, a control unit is required if such environmental pollutants are eliminated and recovered, for example by means of separators, in order to provide rapid, considerable cooling of hot emissions by means of cooling devices and the like before they are fed to the separator. This led to problems in terms of the emission cleaning installation overall becoming complicated and more expensive. Moreover, similar problems are encountered with emission cleaning installations which use bag filters or cyclones.
On the other hand, it has been proposed to initiate decomposition processes by oxidation using a metallic catalyst made from titanium, vanadium, platinum and the like as an additional agent. However, since in this decomposition process by oxidation carried out by the metallic catalyst, the effective temperature of the atmosphere is in the region of 230° C. and also the temperature range which will be effective is narrow, it is necessary for the emissions to be quickly and considerably cooled by means of a cooling device or the like before they are fed to the separator or bag filter or the cyclone, yet at the same time it is necessary for the cooled emissions to be heated and for their temperature to rise again; this makes temperature control of the emissions very troublesome and laborious.
Furthermore, in elimination methods carried out by absorption agents comprising the abovementioned activated carbons or activated charcoals, although in theory it is possible for the low-boiling, gaseous dioxins to be eliminated, it is also necessary for the used waste adsorption agent which has absorbed dioxins to be fed for secondary treatment. Additionally, because the emission temperature is in the region of greater than 150° C., dioxins which have been absorbed with great difficulty become separated from the adsorption agent and are then deposited, or because the adsorption capacity of activated carbons falls, and then the desired elimination by adsorption can no longer be achieved, or because the emission temperature generally fluctuates readily and constantly, various problems are always encountered, involving the dioxins which have been adsorbed at high temperature becoming separated and then dispersed in the emissions once again, etc.